1. Field of the Invention
The invention relates to a piezoelectric actuator, for instance for actuating a mechanical component such as a valve or the like.
2. Description of the Prior Art
It is widely known that by utilizing the so-called piezoelectric effect, a piezoelectric element can be constructed from a material with a suitable crystalline structure. When an external electrical voltage is applied, a mechanical reaction of the piezoelectric element takes place, which depending on the crystalline structure and the regions where the electrical voltage is applied causes a compression or tension in a predeterminable direction.
The aforementioned piezoelectric actuators are often used in the positioning of valves. Among other factors, it must be considered here that their stroke capacity for actuating a valve tappet, for instance, is relative slight. On the other hand, the different thermal expansion of the ceramic comprising the piezoelectric element as opposed to the housing causes problems; the piezoelectric element has only very slight temperature expansion, and the housing, which as a rule is of metal, has a positive temperature expansion, which can cause a drift in the position of the valve tappet without any triggering of the piezoelectric element.
In the conventional way, until now it was possible to reduce such problematic effects only by employing very expensive materials, such as invar, that have a negative temperature expansion. Another way was to connect a material of high temperature expansion in series with the piezoelectric element, but that reduces the rigidity of the system and hence the useful force.
The piezoelectric actuator described at the outset, which can be used for instance to actuate a mechanical component, advantageously has a piezoelectric element, parallel to which, according to the invention, a compensating element is disposed. It is especially advantageous that the piezoelectric element and the compensating element have essentially the same coefficients of temperature expansion, so that given a suitable mechanical mounting of the two elements, the temperature-caused expansions in the piezoelectric element and the compensating element cancel one another out in the effective direction in such a way that an actuating element solidly connected to a pressure plate of the piezoelectric element remains in its position. It is thus possible in a simple way to use a metal housing, for instance of steel, for the piezoelectric actuator as before and to brace the piezoelectric element in the housing in such a way that the compensating element is always solidly connected to the piezoelectric element for the sake of temperature compensation.
In an advantageous embodiment, the piezoelectric element in its effective direction rests with one end against a fixation edge of a housing via a spring and with its other end on another fixation edge of the housing via a pressure plate and a prestressing spring. A spring plate is also present, which is disposed between the piezoelectric element and the spring. According to the invention, the compensating element is additionally disposed on this spring plate and with its other end abuts the housing firmly and moreover is parallel to the piezoelectric element.
In a first refined embodiment, the piezoelectric element comprises a multilayered structure of transversely disposed ceramic piezoelectric layers, which lengthen in the effective direction when an external electrical voltage is suitably applied. The compensating element is likewise constructed of ceramic, with the same coefficients of temperature expansion as the layers of piezoceramic, but this ceramic has no piezoelectric effect. A possible differential expansion between the housing of the piezoelectric actuator and the piezoelectric element that would cause a deflection of the actuating element is thus compensated for via the spring that is located between the spring plate and the fixation edge of the housing.
In a second embodiment, the piezoelectric element likewise comprises a multilayered structure of transversely disposed ceramic piezoelectric layers, which lengthen in the effective direction when an external electrical voltage is applied. The compensating element here is constructed of longitudinally disposed piezoelectric layers, which shorten in the effective direction when an external electrical voltage is applied. Once again, as mentioned above, a possible differential expansion between the piezoelectric actuator housing and the piezoelectric element can be compensated for by means of the same temperature coefficient for the piezoelectric element and the compensating element and by means of the compensation via the aforementioned spring. However, in addition, this embodiment of the invention also makes it possible to lengthen the stroke of the piezoelectric actuator, so that other additional provisions such as a hydraulic coupling can be dispensed with. Because of the lengthened stroke, an otherwise possibly necessary stroke boost can also be dispensed with.
In advantageous refinements of the invention, the piezoelectric element and the compensating element can be constructed in bar form, with a round or rectangular cross section. It is also possible here for the piezoelectric element and the compensating element to comprise hollow cylinders, which are disposed about the axis of the actuating element, to make an overall cylindrical design of the piezoelectric actuator easier.
In a first application of the piezoelectric actuator of the invention, the end of the piezoelectric element by which it rests on the pressure plate, and thus exerts a force on the actuating element, can advantageously be disposed on the far side of the piezoelectric actuator in terms of the effective direction. In that case, the useful force of the piezoelectric actuator is a tensile force.
In a second advantageous application, the end of piezoelectric element by which it rests on the pressure plate, is disposed on the side of the piezoelectric actuator located in the effective direction. In this second case, the useful force of the piezoelectric actuator is a compressive force.